Heart Rate Variability (HRV) Or Your Heart Is Not A Metronome

?? In this Nexus:

?? What is HRV?

?? How HRV is measured?

?? What is a healthy HRV?

? Measuring HRV score with wearable devices

?? The link between HRV and longevity

?? Factors affecting HRV

?? My own experience and tips on HRV

My Nexus Newsletter took one edition of vacation together with me and now we are both back to discuss an important health parameter that I track closely - heart rate variability (HRV). In this Nexus I discuss what is HRV, how it is measured, what is a healthy HRV, why it is important to track it, what is the relationship between HRV, your health and longevity, what factors affects HRV. I also share my personal experience with HRV variations over time. As always, I have mixed the scientific and clinical evidence with some practical tips.

What is HRV?

Lets start from the basics and explain what is HRV for those who are not familiar with the measurement yet. Heart rate variability is the variation in time between each heartbeat. As the title of this Nexus disclaims, unlike a metronome which is highly precise, a healthy heart doesn’t tick evenly. Instead, the time between beats vary slightly and this variability is influenced by the autonomic nervous system. The more it varies - the better.

Autonomic nervous system is a component of the peripheral nervous system that regulates?involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric (1 ). I will discuss the first two as they are mostly related to the HRV.

Activation of the sympathetic nervous system leads to a state of overall elevated activity and attention known as “fight or flight” response. In this process, blood pressure and heart rate increase, glycogenolysis (process by which glycogen is broken down into glucose to provide immediate energy and to maintain blood glucose levels) ensues, gastrointestinal peristalsis ceases, etc.

The parasympathetic nervous system promotes the “rest and digest” processes. Heart rate and blood pressure lower, gastrointestinal peristalsis/digestion restarts, etc.

High HRV indicates a predominance of parasympathetic activity, suggesting a state of relaxation and recovery. Conversely, low HRV indicates sympathetic dominance, often associated with stress and overexertion.

What Does Science Say?

How HRV is Measured?

In scientific literature, HRV is usually expressed in the root mean square of successive differences (RMSSD) between normal heartbeats, which is obtained by first calculating each successive time difference between heartbeats in milliseconds. Then, each of the values is squared and the result is averaged before the square root of the total is obtained. Another measurement is the standard deviation of the RR interval (SDRR), which represents the total HRV, while the RMSSD reflects parasympathetic activity (2 ).

To explain HRV measurement in more human language, let's look at an electrocardiogram (ECG) below, which records the electrical activity of the heart and shows different waves labelled P, Q, R, S, and T. The key interval measured to evaluate HRV is the RR interval, which is the time between two consecutive R waves (the highest peaks in the ECG).

When evaluating HRV, what is being assessed is the variability in the length of these RR intervals over time. Essentially, HRV measures how consistently or variably your heart beats from one beat to the next as shown in the image below (3 ):

What is a Healthy HRV?

As mentioned earlier, your HRV can be high or low, therefore, it is important to define what is healthy for your age and gender. There was a huge clinical study conducted and published in The Lancet Digital Health, which included over 8 million subjects, using Fitbit (now part of Google) devices (4 ). In this cross-sectional study, researchers collected interbeat interval data that was sent to a central database from Fitbit devices during a randomly selected 24 h period. Age, sex, body-mass index, and steps per day in the 90 days preceding the measurement were extracted. The graph below shows different parameters, used to evaluate HRV, I advise you to refer to the first two, RMSSD and SDDR which were described earlier in this Nexus . In this study, researchers has observed a dose-dependent association between physical activity and HRV metrics, with an increase in physical activity associated with increased HRV.

Measuring HRV Score With Wearable Devices

While the measurements above might be too scientific for some, these days it quite easy to measure your HRV score with wearables, and get quite an accurate outcome. HRV can be measured using various devices, including fitness trackers, smartwatches, and specialized HRV monitors. These devices use sensors to detect the time intervals between heartbeats and calculate the variability. The results are usually presented as an HRV score, which can be tracked over time to monitor changes in autonomic function.

Different brands provide a bit different ranges of optimal HRV score based on your age and gender. The study mentioned before used FitBit to measure HRV, below are some examples from Elite HRV (5 ) and ōURA ring users (6 ). It is useful to cross-check where you are with your HRV scores and how you can improve.

The Link Between HRV and Longevity

You don't have to be a scientist to look at all the graphs above and state the obvious - HRV decline with age, thus, it is directly associated with longevity and healthspan. The association between HRV and disease states is well-documented, with reduced HRV being linked to increased morbidity and mortality across various conditions.

Cardiovascular health. In cardiovascular health, low HRV is a strong predictor of adverse events such as myocardial infarction, heart failure, and sudden cardiac death. This association is primarily due to the impaired autonomic regulation seen in these patients, where there is often a dominance of sympathetic over parasympathetic activity, indicating a reduced ability to respond to physiological stressors. For example, patients with heart failure often exhibit reduced HRV, which correlates with poorer prognosis and higher mortality rates (7 ,8 ).

Critical health conditions. Additionally, HRV serves as a valuable prognostic tool in critically ill patients and those with sepsis (a life-threatening condition that arises when the body's immune response to infection causes widespread inflammation, leading to tissue damage, organ failure, and potentially death). In these populations, lower HRV has been shown to predict adverse outcomes, including increased mortality, due to the autonomic nervous system's inability to maintain homeostasis under severe physiological stress.

Chronic conditions. The prognostic value of HRV extends to other chronic conditions such as diabetes and psychiatric disorders, where altered autonomic function is a common feature. For instance, in diabetic patients, reduced HRV is associated with a higher risk of cardiovascular complications and all-cause mortality. Similarly, individuals with depression or anxiety often show decreased HRV, reflecting heightened sympathetic activity and reduced parasympathetic tone, which are linked to worse mental health outcomes and increased cardiovascular risk (7 ,9 ).

Factors Affecting HRV

HRV is influenced by a variety of factors, which can be categorized into physiological, genetic, lifestyle, environmental, and pathological factors. Each of these categories contributes to the complex regulation of autonomic function reflected in HRV.

Several physiological factors are intrinsic to an individual's HRV. Age is a significant non-modifiable factor, with HRV generally decreasing as one ages due to a decline in autonomic nervous system function and reduced vagal tone. Gender differences also exist; women generally have higher HRV than men, which may be attributed to hormonal differences influencing autonomic regulation. Physical fitness levels and overall health status are other physiological factors, with healthier and more physically fit individuals typically exhibiting higher HRV (9 ).

Genetic predisposition can significantly influence HRV. Variations in genes related to the autonomic nervous system can affect how HRV is regulated. For instance, polymorphisms in genes coding for adrenergic receptors and enzymes involved in neurotransmitter metabolism can lead to differences in autonomic function and HRV among individuals. Understanding these genetic influences can help in identifying individuals at higher risk for conditions associated with low HRV and in developing personalized interventions to improve autonomic function (9 ,10 ).

Lifestyle factors significantly impact HRV (7 ,10 ), as described below:

• Regular physical activity is associated with increased HRV, indicating enhanced autonomic function and cardiovascular health. Exercise promotes parasympathetic activity, which is beneficial for heart health. In contrast, a sedentary lifestyle is linked to lower HRV and poorer autonomic regulation.
• Diet also plays a crucial role. Diets rich in omega-3 fatty acids and low in processed foods are associated with higher HRV.
• Chronic stress is another critical lifestyle factor that negatively affects HRV by increasing sympathetic nervous system activity and reducing parasympathetic activity.
• Sleep quality and duration are also vital as poor sleep reduces HRV by impairing autonomic balance.

Environmental factors, including ambient temperature, noise, and altitude, can influence HRV. Cold environments tend to increase HRV by enhancing parasympathetic activity, whereas extreme heat can reduce HRV by stressing the cardiovascular system. Noise pollution, particularly chronic exposure to high levels of noise, has been shown to decrease HRV, reflecting increased stress and sympathetic dominance. High altitude can also affect HRV due to hypoxia, which typically reduces HRV as the body adapts to lower oxygen levels (7 ,10 ).

As described in the earlier chapter about the link between HRV and longevity, pathological conditions also greatly impact HRV, often leading to a reduction in variability.

My Own Experience and Tips on HRV

I would like to share some of my own experiences on HRV and some practical tips. I track HRV on daily basis for as long as I have my wearable device - Garmin Fenix 6S Sapphire smartwatch, so for almost 4 years now. My insights would be:

1. HRV is highly variable day by day. It very much depends on how you coupe with maintaining your healthy lifestyle, it can go from hero one day to kind of zero the next day.
2. For me sleep quality is a number one predictor of HRV. If I do exercise too late in the day, if work until very late hours and go to sleep at 1 or 2 AM, if I sleep too short in general, or if I consume some alcohol too late in the day, I know my sleep quality will be bad and my HRV will go down that night. I have done some screenshots of my daily HRV scores from different periods and circumstances (see below). The A screenshot represents one of my worst HRV scores in the past year, and during that week I was heavily traveling, waking up crazy early for my flights, not sleeping enough each day. The results are obvious.
3. Lazy me - decreased HRV. If there is a week when I am not physically active at all, I can see how my HRV starts going down day by day. Consistency is key in basically anything related to your health and longevity.
4. A state of any disease automatically reduces HRV. Those who are close to me know that back in May 2023 I got sick with chickenpox for the second time in my life (!!!), which happens very rarely. The B screenshot represents my HRV during that worst week of the infection, when for 5 days I felt like after my first Astra Zeneca COVID-19 shot (meaning horrible), just for an extended period of time, adding the crazy itching on top of the feeling "I am going to die".
5. To improve your HRV score you need a higher intensity physical activity. When I sleep well and do some physical activity, my HRV scores stays somewhere between 50 and 55, which is healthy for my age. However, my Garmin watch was constantly telling me that I need some higher intensity training, thus I started running back in June this year. The C screenshot represents my HRV scores after running for around 3 weeks, 3-4 times a week, short but increasing distances (from 2.5 gradually increasing to 5 km). As you can see one night I even achieved HRV score of 70, which is above average for a 20 year old woman. The conclusion - you need to work harder to improve it.
6. To keep your HRV scores high it requires continuous effort. As this parameter can vary so much day by day, it means that it is not something you are going to build and can get lazy afterwards or put just minimal efforts to maintain it. It is a lifelong effort that you need to make, which as described earlier provide significant health benefits.

I hope you have learned something new about the heart rate variability and will be checking it more frequently, in order to identify the patterns how your body reacts to changing environment and circumstances. My Nexus is not planning to take more holidays this summer, therefore, expect to hear more about health, longevity and how to prolong your healthspan in 2 weeks!